I'm pretty sure I've done this and that they stack - they are separate sources of damage from two different players? I don't think it matters whether they are necrotic or not. I have definitely used two spirit guardian in some game...can't recall if it was BG3 or Solasta. And that did the expected cumulative damage. It's a nasty combo for enemies, especially if you upcast it and they are weak to radiant (or its stop regeneration etc) It doe require concentration which is an issue, since that is then all you can maintain.
